Jump to Navigation Quaternary Fault and Fold Database of the United States As of January 12, 2017, the USGS maintains a limited number of metadata fields that characterize the Quaternary faults and folds of the United States. For the most up-to-date information, please refer to the interactive fault map. Darrington-Devils Mountain fault (Class A) No. 574 Last Review Date: 2016-11-29 citation for this record: Johnson, S.Y., Blakely, R.J., Brocher, T.M., Haller, K.M., and Barnett, E.A., compilers, 2016, Fault number 574, Darrington-Devils Mountain fault, in Quaternary fault and fold database of the United States: U.S. Geological Survey website, https://earthquakes.usgs.gov/hazards/qfaults, accessed 12/14/2020 03:03 PM. Synopsis The Darrington-Devils Mountain fault zone extends from southwest of Darrington to west of Whidbey Island where it may connect with the Leach River fault (Dragovich and DeOme 2006 #7643) or San Juan faults on Vancouver Island, B.C. (Johnson and others, 2001 #4749). The Devils Mountain fault terminates in northwest-trending en-echelon folds and faults, a map pattern strongly suggesting that it is a left-lateral, oblique- slip, transpressional structure (Johnson and others, 2001 #4749). Aeromagnetic anomalies coincide with both the trace of the Darrington- Devils Mountain fault and en-echelon structures (Blakely and others, 1999 #4747; Johnson and others, 2001 #4749). Quaternary strata are deformed on nearly all seismic-reflection profiles crossing the fault in the eastern Strait of Juan de Fuca, and onshore subsurface data suggest offset of upper Pleistocene strata across the fault (Johnson and others, 2001 #4749). Pleistocene strata across the fault (Johnson and others, 2001 #4749). Trenching confirms at least one Holocene coseismic surface rupture (Personius and others, 2014 #7644). Name Geologic mapping by Norbisrath (1939 #4760) and Hobbs and Pecora comments (1941 #4758) documented a west-trending fault zone in the western Cascade Range foothills east of Mount Vernon, Washington. Lovseth (1975 #4759) conducted subsequent mapping and named this zone the Devils Mountain fault, as used in later reports (e.g., Whetten, 1978 #4764; Johnson and others, 2001 #4749). This compilation follows Tabor (1994 #4762) and subsequent reports (Haugerud and others, 2003 #6211;per 2014) that link the Devils Mountain fault with the Darrington fault zone of Vance and others (1980 #6242) farther east and uses the name Darrington- Devils Mountain fault zone. County(s) and SKAGIT COUNTY, WASHINGTON State(s) SAN JUAN COUNTY, WASHINGTON (offshore) SNOHOMISH COUNTY, WASHINGTON ISLAND COUNTY, WASHINGTON Physiographic PACIFIC BORDER province(s) CASCADE-SIERRA MOUNTAINS Reliability of Good location Compiled at 1:24,000 scale. Comments: Location of fault from GER_Seismogenic_WGS84 (http://www.dnr.wa.gov/publications/ger_portal_seismogenic_features.zip, downloaded 05/23/2016) attributed to Dragovich and others (2002 #7638, 2003 #7639, 2003 #7640, 2004 #7641, 2005 #7600) and Dragovich and DeOme (2006 #7643) Geologic setting The Darrington-Devils Mountain fault is located at the northern boundary of the northward-migrating portion of the forearc region of the Cascadia convergent continental margin (Wells and others, 1998 #4742; Miller and others, 2001 #4732). Geodetic studies (e.g., Khazaradze and others, 1999 #4734) indicate about 4–5 mm/yr of north-south crustal shortening in western Washington, some of which is accommodated by slip on the Darrington-Devils Mountain fault zone (Johnson and others, 2001 #4749). Largely based on seismic tomography (Brocher and others, 2001 #4718; Ramachandran, 2001 #4761; Zelt and others, 2001 #4767), the western part of the Darrington-Devils Mountain fault zone in the eastern Strait of Juan de Fuca has been proposed to form the southern limb of a structural pop-up cored by the San Juan Islands (Brocher and others, 2001 #4718). Onland to the east, the fault forms the northern boundary of the Tertiary- to-Quaternary Everett basin (Johnson and others, 1996 #4751; 2001 #4749). Length (km) 123 km. Average strike N81°W Sense of Left lateral, Thrust movement Comments: Numerous geologic relationships and seismic-reflection profiles indicate north-side-up thrust/reverse faulting (Lovseth, 1975 #4759; Whetten and others, 1988 #4766; Johnson and others, 1996 #4751; 2001 #4749). The distribution of geologic units on opposite sides of the fault zone (Lovseth, 1975 #4759; Whetten, 1978 #4764) and the geometry of en-echelon faults and folds (Johnson and others, 2001 #4749) suggests a component of left-lateral slip. Trenching studies suggest right-lateral displacement is the predominate sense of slip in the late Holocene (Personius and others 2014 #7644). Dip 45–90° N Comments: Johnson and others (2001 #4749) reported north dips ranging from 45° to 75° N (61°±10°) based on interpretation of seismic-reflection profiles in the eastern Strait of Juan de Fuca, with the steepest dips to the east in profiles flanking Whidbey Island. Whetten and others (1988 #4766) include a cross section from the Cascade Range foothills in which the fault zone of the Darrington-Devils Mountain fault (3-km-wide zone of 6 faults) is interpreted as vertical and faults in trench exposures are near vertical (Personius and others, 2014 #7644). Paleoseismology Johnson and others (2001 #4749) mapped and described the western part studies of the fault zone across Whidbey Island and the eastern Strait of Juan de Fuca using seismic-reflection profiles, high-resolution aeromagnetic data (Blakely and others, 1999 #4747), stratigraphic analysis of water well logs, and outcrop mapping. Lovseth (1975 #4759) and Marcus (1980 #4765) conducted structural and stratigraphic studies along the fault zone in the Cascade Range foothills. Site 574-1, Lake Creek study site (Personius and others 2014 #7644) is located southeast of Mount Vernon, Washington. This study consisted of three-dimensional trenching of prominent fault scarps and coring of the adjacent wetland (Boomer Marsh) that results in a record of one and possibly two paleoearthquakes on the central Darrington–Devils Mountain fault zone since deglaciation of the Puget Lowland in the latest Pleistocene. The Aplodontia and Springboard trenches were about 25 m apart and crossed a 1-m-high south-facing scarp. The trenches exposed Vashon Stade glacial sediments (14,725±470 radiocarbon yr BP apart and crossed a 1-m-high south-facing scarp. The trenches exposed Vashon Stade glacial sediments (14,725±470 radiocarbon yr BP Dragovich and DeOme, 2006) buried by Holocene sediments. The Boundary trench, about 125 m west of the Springboard trench, crossed a 1.5-m-high scarp. The trench exposed highly sheared bedrock eroded into and overlain by a sequence of glacial-fluvial, glacial-lacustrine, and till sediments deposited at the basal glacier-bedrock contact. Offset determined from faulted glacial features is 2.25±1.1 m of right-lateral displacement and 0.6±0.1 m north-side-up vertical displacement or right- lateral oblique net slip of 2.3±1.1 m plunging 14° west on a vertical fault striking 286°. In addition, Personius and others excavated two trenches across scarps that cross Whitman bench; the trenches did not expose evidence of tectonic deformation so Personius and others (2009 #7644) concluded that the scarp is a glaciofluvial channel margin. Geomorphic The Darrington-Devils Mountain fault coincides with a prominent 30-km- expression long topographic lineament in the western foothills of the Cascade Range (fig. 4 in Johnson and others, 2001 #4749). This lineament coincides with segments of several stream drainages and with the axis of the elongate, 4.5-km-long Lake Cavanaugh. Detailed geologic mapping from Whidbey Island on the west to near Darrington on the east document abundant evidence of Quaternary displacement along the fault zone (see summary in Dragovich and DeOme, 2006 #7643). Age of faulted Eocene strata of the Chuckanut Formation and the Oligocene rocks of surficial Bulson Creek (Marcus, 1980 #4765; Whetten and others, 1988 #4766) are deposits juxtaposed along the Darrington-Devils Mountain fault in the Cascade Range foothills. On offshore seismic-reflection data, inferred Quaternary strata are both folded and faulted by strands of the Darrington-Devils Mountain fault (Johnson and others, 2001 #4749). Dragovich and others (2002 #7638, 2003 #7639, 2003 #7640, 2004 #7641, 2005 #7600) and Dragovich and DeOme (2006 #7643) locally show faulted Quaternary deposits. Historic earthquake Most recent late Quaternary (<130 ka) prehistoric deformation Comments: Analysis of water-well logs from northern Whidbey Island indicate offset of the late Quaternary (80–125 ka; stage 5 interglacial) Whidbey Formation (Johnson and others, 2001 #4749). Trench excavations revealed evidence of a single earthquake, radiocarbon dated to 2 ka, but extensive burrowing and root mixing of sediments within 50– 100 cm of the ground surface may have destroyed evidence of other earthquakes; coring of the nearby wetland results in possible evidence of an older Holocene surface rupture (Personuis and others, 2014 #7644). Recurrence 6 k.y. (<8 ka) interval Comments: Two recognized earthquakes, dated at 1.9±0.4 ka and 8.1±0.1 ka, yield a single recurrence interval of about 6 k.y. (Personius and others, 2014 #7644). Slip-rate Less than 0.2 mm/yr category Comments: Personius and others (2014 #7644) report an average slip rate of 0.14±0.1 mm/yr based on 2.3±1.1 m of right-lateral oblique slip (on a vertical fault) that postdates deglaciation of the Lake Creek site, about 16±0.5 ka. However, they consider this a minimum because slip on the southern strand of the fault near the trenches and possible other unmapped fault traces is not included. Stratigraphic analysis of well logs from northern Whidbey Island suggests a vertical displacement rate of about 0.05–0.30 mm/yr (preferred rate is 0.16 mm/yr) over the last 80 ka. Offshore seismic-reflection profiles suggest minimum Quaternary, vertical rates of 0.03 to 0.13 mm/yr.